10 Days trip to Japan

Hi Daniel-san,

I am planning to visit Japan in April 2023, below is my plan itinerary:
3 Apr - arriving Narita Airport 7:30am, Narita to Osaka
4-6 Apr - around Osaka
7 Apr - Osaka to Kyoto
8 Apr - Around Kyoto
9 Apr - Kyoto to Tokyo
10-11 Apr - Around Tokyo
12 Apr - Tokyo - Narita

Question:
1. Should I be getting 7 days instead of 14 days, since it would be sufficient to bring me back to Tokyo on day 7 (9 Apr). (Just trying to be cost effective as I will be travelling with family of 6)
2. I understand that Shinkansen has luggage size limitation and I believe I will need to prebook seat to bring in bigger (>160cm) size luggage. I also understand that I need to collect the ticket at Narita airport, get Narita Express to Tokyo to catch on Shinkensen to Osaka. How do I know which slot should I be booking for Shinkensen? What happened by the time I reach Tokyo, my booking slot has passed?
3. Has Narita Express have any luggage size restrictions also?

Thanks & regards,
WM